Multiple Access
Multiple access is a fundamental problem in wireless networks, such as cellular systems, ad hoc networks, sensor networks, cooperative and collaborative communication networks, relay networks, and the like. Multiple access enables multiple contending transceivers to access the network, i.e., transmit and receive packets. The tranceivers are also referred to as nodes, users or mobile stations (MS). A large number of protocols are known to solve the multiple access problem. The protocols use either contention-free or contention-based access.
Contention Free Access Protocols
In the contention free access case, each node is allocated a reserved time slot, frequency, and/or spreading code, which the node can use to transmit packets with little or no interference. The allocation is typically performed by a centralized radio resource management entity, such as a base station (BS), access point, or ‘receiving’ node. However, the efficiency of such schemes can he low, especially when the traffic is ‘bursty’. Furthermore, contention-free access schemes usually require centralized control, which in turn necessitates an overhead that makes those schemes less desirable for handling a network: with a larger number of nodes, e.g., hundreds or thousands.
Contention Based Access Protocols
Contention based access protocols can be implemented in a distributed way. Each node transmits whenever the node has packets to send. This can lead collisions, in which packets transmitted concurrently by different nodes overlap and interfere with each other at the receiving node (receiver).
The design of multiple access schemes has traditionally attempted to ensure that each node has a fair chance of accessing the channel, on average. However, in problems such as multi-user diversity in the uplink of a cellular system, the aim of the multiple access scheme changes to quickly selecting, at any point in time, the node with the highest channel gain to the BS.
In one scheme, a pilot signal is broadcast periodically by the BS to all mobile stations (MSs) to enable each MS to determine its channel gain and feed the channel gain back to the BS. Then, the BS schedules a downlink or uplink transmission for the best node.
Another example that arises in a very different setting is relay selection in cooperative communication systems. In that setting, the source node needs to select the best relay node to forward its message to the destination node. Notice in all the above examples that: local channel knowledge gives the node an estimate of its relative importance and usefulness.
A common assumption in the design of multiple access schemes is that when packets interfere with each other, none of the colliding packets can be decoded properly. However, that collision model is a coarse and pessimistic way to model a wireless physical layer that handles interference. So long as the power of one received signal is sufficiently stronger than the interference power, the receiver could perhaps decode the stronger signal. This statement is valid even if no special measures for interference mitigation, such as multi-user detection or smart antennas, are used.
MPR
A generalization of signal acquisition and decoding is called multi-packet reception (MPR). Methods for achieving MPR include space-time coding, multiple input multiple output signaling, spread spectrum modulation, frequency hopping, and multiple access coding. Signal acquisition is exploited in many systems, such as Aloha networks, IEEE 802.11 compliant systems, Bluetooth radios, and cellular systems. The collision model ignores the fact that the powers of received signals are often asymmetric due to different path losses or different transmitted powers of the nodes—both of which actually aid signal acquisition.
Some methods exploit local channel knowledge to improve the efficiency of contention-based multiple access. One channel-aware Aloha scheme incorporates channel knowledge to control channel access. Each node transmits only if its channel gain exceeds a system-determined threshold. An opportunistic Aloha (O-Aloha) protocol sets the probability of transmission as a function of local channel knowledge, which is only required to be known locally at the respective contending transmitters. Thus, transmitters with a high channel gain are more likely to attempt a transmission.
Note, none of the above multiple access schemes attempts to adjust the power of the transmitted signal.